There have been offered various illumination optical systems used in projectors such as a LED (Liquid Crystal Display) projector and a DLP (Digital Light Processing) projector.
JP 2010-86815A (hereinafter, Patent Literature 1) discloses an illumination optical system that irradiates a fluorescent body with excitation light emitted from a laser light source to acquire illumination light from the fluorescent body. In the illumination optical system disclosed in Patent Literature 1, the irradiation spot of the excitation light is periodically moved over the fluorescent body by rotating or moving a substrate on which the fluorescent body is disposed. This may prevent the reduction of light emission efficiency caused by a temperature increase.